Diane Mendoza sits at her kitchen table, fingering a heart-shaped pendant that hangs on a heavy silver chain around her neck. At 44, she has five children. Those children have three different fathers.

"When I married my first husband and had my first child, I thought it was for life," she says, her voice jagged from years of smoking and hollow from decades of disappointment. "I hated the fact that I had never known my father. I never wanted that for my children. But sometimes you rush into decisions and they don't always pan out right."

Diane and her husband were divorced within six years. A mother-of-two, and still only 24, she plunged straight into another disastrous relationship and had two more children.

Those children, the three eldest boys in particular, grew up without boundaries; often high on drugs, they spent their time shoplifting, vandalising property, daubing buildings with graffiti and racing bangers. All three boys were expelled from school.

"Mark [the second eldest] wasn't too bad," Diane says, without a hint of irony. "He made it to the second year of seniors before he was excluded."

It is exactly this sort of lifestyle that a senior judge condemned in an uncompromising speech in Parliament last month. Mr Justice Coleridge, a leading family lawyer, said mothers and fathers who have failed to commit to each other, and engaged in a game of "pass the partner", have left millions of children "scarred for life". He called for a national commission to establish marriage as the "gold standard" for relationships.

Now, a new report to be published this week by the Social Justice Policy Group, a think tank committed to tackling poverty, has drawn similar conclusions, as revealed in today's Sunday Telegraph.

The report, Every Family Matters, written by the think-tank's founder and former Conservative Party leader Iain Duncan Smith MP, calls for an overhaul of the law to "put marriage" at the heart of family life.

Couples wanting to divorce should face a compulsory three-month "cooling-off period" before proceedings could start, according to Duncan Smith. This would enable them to "find out the implications" of a split – from how it will affect their finances, to the impact it will have on their children. "The three-month period will endeavour to save saveable manages," says Duncan Smith.

In many voters' eyes, family breakdown is directly related to the rise in thuggery, drug abuse and street violence. The number of young people stabbed to death in the past three years suggests a street life reminiscent of William Golding's Lord of the Flies. The casual intimidation and vandalism by groups of teenagers roaming around neighbourhoods terrifies residents up and down the country.

According to Conservative leader David Cameron, these are symptoms of a society that is broken; in which stable two-parent families are becoming the exception; where individual rights have blunted our sense of duty and responsibility; and where successive generations of children face a life devoid of hope or dignity.

There is an underclass, he believes, that wields a disproportionate influence in terms of the crime it spawns and the huge amount of public money it soaks up – mostly in welfare benefits and funding for the criminal justice system. (Over the past 10 years the cost of policing has risen by 40 per cent and that of working-age benefits by 25 per cent.) And, despite Gordon Brown's much-vaunted ambition to create a more equal society, Britain's underclass has grown since Labour came to power.

The Centre for Social Justice estimates there are one million more people living in ''severe poverty'' (defined as earning 40 per cent of the average national wage) than in 1997.

"Young kids in many cities are running riot. There's been a rise in gang culture and in drug culture," says Philippa Stroud, the centre's executive director, when I meet her in her cramped office in central London. "We have 30,000 children every year leaving school with no educational qualifications. We also have the highest levels of teenage pregnancy in Europe."

According to the centre, there are five poverty ''drivers'': family breakdown; welfare dependency; educational failure; addiction to drugs and alcohol; and serious personal debt. But the key problem is the breakdown in marriage over the past 40 years.

Figures published this year by the Office for National Statistics showed there were only 231,000 marriages in England and Wales in 2007, the lowest total in 112 years. If trends continue, nearly half of those are likely to end in divorce. Countless studies show that children who grow up in two-parent families do far better, on average, than those brought up by a single parent. This applies to every sphere of life – from jobs to health and wellbeing. But this fact has never been acknowledged by the Left.

"This Government has very clearly sent a message that a marriage and a relationship are completely equal and has almost created an environment where to say anything else is viewed as being small-minded," says Stroud. "And when you look at the evidence, you see how destructive that message is."

Stroud is softly-spoken, like the founder of her organisation, but her anger towards the liberal establishment is plain. "They don't even bother keeping stats now on who is married and who is not. It's a real sign that we do not value the difference. Policy-makers refuse to admit that marriage makes any significant contribution to society."

This is most evident in the tax and benefit system, which provides no financial incentive to get married or to stay married. A couple on £18,000 a year who live in the same house pay a ''penalty'' in reduced benefits of £8,588 a year. When they live together, they are assessed for benefits on their combined income. If the man moves out, his wife and children are assessed as eligible for benefits on the basis of her income alone.

Diane Mendoza knows that all too well. She is married to her second husband, Steve, but says she has had to pretend to be single to keep her benefits. A few years ago, when she was in danger of losing her council house because of the yobbish behaviour of her children, she admits she lied to her solicitor about her marital status.

"Every time I sat [in his office], I took my wedding ring off. I had a mark there because of the sun so I had to sit there like that" – she clasps her right hand over her left. "It's in the back of your head the whole time, that you could be done for that, but that's what I had to do, because I could have lost my home."

In fact, surveys show that the vast majority of women on estates like Monks Hill in Croydon, where Diane lives, want to marry. Like most people, they want the stability of a long-term relationship and help with the children. In fact, hardly any couples on Monks Hill stay together. Poverty is a big factor. When Diane had her first child, she and her husband didn't have enough money for baby's milk. "The struggle was unbelievable," she says.

It drove a wedge between the couple – but to make matters worse, Diane could not call upon any relatives to help with childcare.

"My mum is not like a granny," she says, giving a shrug. "Not the type of nanny where you can say, 'Can you look after the kids for a couple of hours?' And my brothers and sisters have got their own problems."

Diane's parents divorced while she was a baby and her father played no part in her life. She was brought up by a stepfather who was on incapacity benefit and never worked and a mother who was too absorbed in her own problems to care adequately for her children.

Neither Diane nor her siblings completed school. "I was told I was good for nothing," she says flatly. By the time she was 16, her only ambition was to become a mother.

"I'd been with my fella for nine months, which felt like a very long time at that age and so we went ahead and had a baby. There was no thought of what we were going to do once we had a baby, it was just a maternal drive within me that craved a child."

Diane and her husband were simply not prepared for marriage or parenthood. "People don't understand how to resolve conflicts," says Stroud. "They have little concept of the general give and take of normal relationships. Part of it is an education thing. At school you learn to sit in a classroom, engage with a teacher, to listen to somebody and to absorb information.

"All of those things are relationship skills. And if there are no examples of any successful relationships in your family or your community, then that puts you at an enormous disadvantage. If you are a product of a broken home you reproduce what you have experienced. It may vary a little, but not dramatically."

Prior to its Every Family Matters report, the Centre for Social Justice had been putting forward many proposals to build stronger families. Most important have been suggested changes to the welfare system to give low-income families an incentive to get married and stay together. Relatively small sums are involved – Duncan Smith's think tank suggested topping up the Working Tax Credit offered to families by £32 a week – but they would make a difference to couples on low incomes.

The organisation also recommends a national programme of relationship and parenting classes to help people who were never taught these skills by their parents. These are run in Monks Hill and other parts of Croydon by Jane Carey, a cheerful, fresh-faced 47-year-old.

On the day I visit, it is the final session in a six-week course, "Time Out For Teenagers". Six parents, five women and one man, sit in a circle in a church hall. Some ''Ground Rules'' have been written out on a piece of paper and stuck on the wall: "Mobiles on, but please answer outside. Respect each other's opinions. What is said in here stays in here." And, "It's OK to be upset."

The course has taught them some basic strategies for coping with teenagers. They have learnt to set boundaries, empathise and put time aside for ''focused activities'' with their children – such as swimming, cooking, walking or even watching television (as long as they discuss the programme afterwards).

And what makes teenagers angry? Suggestions include: "Unfairness"; "Not getting their own way''; "Getting up in the morning"; and "Unfairness" (again).

Jane writes them all on a large piece of paper pinned to an easel, with a list of things that make the situation worse: being hot, being tired or being hungry. As the session goes on, the group discusses ways of avoiding conflict and controlling anger. They are nothing spectacular – "Count to ten"; "Breathe deeply"; "Don't get drawn into a debate" – but the participants welcome them with enthusiasm.

Nevertheless, I wonder how long they will stick to what they've been taught. Does a person's parenting improve over the long term?

"Yeah, it does," says Jane, when I speak to her after the class. "I would say the main improvement is confidence that they can do this job. That they now have some strategies to know what to do.

"In this era of children's rights, when we are no longer allowed to smack our children, parents no longer know what they can do. And with the breakdown in extended families and the breakdown in marriage, no one really knows where to get help."

Like Philippa Stroud, Jane does not deafen you with her opinions, but she has an unwavering dedication to her cause. "A lot of kids we meet actually just need attention," she says, locking the door to the hall. "They need to feel valued. If you changed that. If parents could get it right in the first place… That's the way to turn societies like Monks Hill around."